Consumers have become conscious of environmental problems. This has resulted in the popularity of battery-powered electric vehicles. Japanese Laid-Open Patent Publication No. 9-161898 describes one example of an electric vehicle. The electric vehicle includes an inlet that can be connected to a power plug extending from, for example, a household commercial power supply. Power is transmitted from the commercial power supply to the vehicle through the power plug connected to the inlet to charge the vehicle battery.
The charging of a battery for an electric vehicle requires a longer time than filling gasoline into a gasoline vehicle. Thus, an electric vehicle may often be left unattended with the power plug connected to the inlet. As a result, the power plug may be removed from the vehicle and connected to another electric vehicle to steal electricity. Moreover, the power plug may be stolen.
Japanese Patent No. 4379823 describes a plug lock device that locks a power plug to an inlet and restricts unauthorized removal. The power plug lock device unlocks the power plug in cooperation with the unlocking of vehicle doors.
Further, smart systems are installed in recent vehicles. In a smart system, wireless communication is performed between the vehicle and an electronic key carried by a user. The smart system locks or unlocks the vehicle doors and permits the starting of the engine only when the wireless communication is established. Japanese Laid-Open Patent Publication No. 2005-194799 describes one example of a smart system. The smart system intermittently transmits wireless signals on the low frequency (LF) band to form a communication area around the vehicle. When the electronic key enters the communication area and receives an LF band wireless signal, the electronic key transmits a wireless signal on the ultrahigh frequency (UHF) band. When the vehicle determines that the received UHF band wireless signal is valid, the vehicle permits the unlocking of the doors or the like.